The strong cell surface antigens induced in polyoma virus transformed mammalian cells represent one of the characteristics most closely associated with transformation by this virus. Through their interaction with the immune system of the host these antigens become a major factor in limiting the tumorgenicity of the transformed cell. Cell surface antigen changes associated with polyoma virus transformation can be detected by in vitro methods and by in vivo methods. It is proposed to use assays of the in vitro type to explore the chemical nature of these antigens by such approaches as detergent solubilization followed by fractionation by precipitation, centrifugation, chromatography and electrophoresis and by determination of their stability to chemical modification and enzymatic digestion. By these means at least partial purification and general chemical characterization of these materials should be achievable and should indicate something of the specific molecular changes induced at the cell surface by polyoma transformation. Secondly, solubilized materials active in vitro would be tested for in vivo transplant rejection activity. This should lead to general categorization and hopefully specific identification of those compounds with which the strong transplant rejection is associated. It should also provide a means of better determining the relationship between antigens detected in vivo versus those detected in in vitro assays. The relative enzymatic and chemical stability and species distribution of these two catgories of antigens would be explored to further investigate their hypothesized identity. Finally, the question of whether these antigens are viral coded would be approached in such experiments as looking for exposure of antigen-like activity as a result of protease treatment and by determination of species distribution of induced antigens. Establishment of whether or not the antigens are viral in origin is of importance in understanding how these antigens are generated.